1. Field of the Invention
The present invention relates to a resonant circuit, a filter circuit, and a multilayered substrate which are applicable to a broadband wireless system.
2. Description of the Related Art
Recently, development of wireless systems using broadband has been considered. In a wireless system using broadband, a band-pass filter (BPF) which filters required signals and unnecessary signals should match the broadband and preferably has a low insertion loss.
Examples of a resonant circuit, a filter circuit, and a multilayered substrate for being used for broadband which are based on general technology will now be described.
FIG. 16 is a diagram showing a general resonant circuit. FIG. 17 is a diagram showing a general filter circuit. FIGS. 18 and 19 are diagrams showing the frequency characteristics of a general filter circuit. FIG. 20 is a perspective view showing an external shape of a general filter element. FIG. 21 is a schematic plan perspective view showing a general multilayered substrate. FIG. 22 is a schematic side perspective view showing a general multilayered substrate. FIG. 23 is a schematic exploded perspective view showing a general multilayered substrate. FIG. 24 is an equivalent circuit of a filter circuit in a general multilayered substrate. FIG. 25 is a diagram showing the frequency characteristic of a filter circuit on a general multilayered substrate.
As shown in FIG. 16, the resonant circuit based on the general technology which is used in the BPF includes transmission lines 213 and 214 which are connected in series between input and output terminals 211 and 212, a transmission line 215 connected to a junction of the transmission lines 213 and 214 and the ground, and a capacitor 216 connected between the transmission line 215 and the ground.
A filter circuit 220 which forms a BPF having a pass band of 6 to 10 GHz using the resonant circuit 210, as shown in FIG. 17, includes a capacitor 221 interposed between the input terminal 211 and the input transmission line 213 and a capacitor 222 interposed between the output terminal 212 and the output transmission line 214.
A frequency characteristic of the filter circuit 220 is as shown in FIGS. 18 and 19. In each of the figures, a curve A is an attenuation characteristic curve, and a curve B is a reflection amount characteristic curve. In addition, a vertical axis represents an attenuation amount and a reflection amount in units of dB, and a horizontal axis represents a frequency in units of GHz. As the attenuation characteristic curves A in the figures indicate, the attenuation amount has local maximal values at frequencies of 5 GHz, 22.5 GHz, and 43.5 GHz. As the reflection amount characteristic curves B in the figures indicate, the reflection amount has local minimal values at frequencies of 8.5 GHz and 31.5 GHz.
An example of an external shape of a filter element 230 is shown in FIG. 20, when the filter circuit 220 is formed as a circuit element. The filter element 230 is formed by stacking the filter circuit 220 inside a layered product 231 having a rectangular parallelepiped shape which is made mainly of dielectric ceramics. In addition, on a surface of the layered product 231, external terminal electrodes 232 to 235 are formed. Here, an external terminal electrode 232 corresponds to the input terminal 211, and an external terminal electrode 233 corresponds to the output terminal 212. In addition, external terminal electrodes 234 and 235 are earth terminals.
The shape and disposition of a conductor pattern are shown in FIGS. 21 to 23, when another exemplary BPF to which the filter circuit 220 is applied is formed inside the multilayered substrate.
In FIGS. 21 to 23, a reference numeral 240 denotes a multilayered substrate which is made mainly of dielectric ceramics. Inside the multilayered substrate 240, a filter circuit 290 to be described later (shown in FIG. 24) is formed by stacking multiple conductor patterns. Described in more details, in the multilayered substrate 240, on both sides of a portion in which the filter circuit 290 is formed, earth conductor patterns 241 and 242 parallel to each other are disposed, respectively. In addition, between the earth conductor patterns 241 and 242, an earth conductor pattern 243 having a predetermined size is formed to be parallel to the earth conductor patterns 241 and 242. The earth conductor pattern 243 is electrically connected to the earth conductor patterns 241 and 242 through a plurality of via conductors 244. In addition, between the earth conductor patterns 241 and 242, an input conductor pattern 250, an output conductor pattern 260, a resonance conductor pattern 270, and a connection conductor pattern 280 are formed.
The input conductor pattern 250 is formed to be parallel to the earth conductor patterns 241 and 242. The input conductor pattern 250 includes an input strip line 251, a capacitor electrode 252 connected thereto, a strip line 253 of which one end is connected to the capacitor electrode 252, and a capacitor electrode 254 which is connected to the other end of the strip line 253. Here, only the capacitor electrode 254 is disposed to face the earth conductor pattern 243.
The output conductor pattern 260 is formed to be parallel to the earth conductor patterns 241 and 242 and to be on a same side as the input conductor pattern 250. The output conductor pattern 260 includes an output strip line 261, a capacitor electrode 262 connected thereto, a strip line 263 of which one end is connected to the capacitor electrode 262 and which is disposed parallel to the strip line 253, and a capacitor electrode 264 which is connected to the other end of the strip line 263. Here, only the capacitor electrode 264 is disposed to face the earth conductor pattern 243.
The resonance conductor pattern 270 includes a strip line 271 which is interposed between the input and output conductor patterns 250 and 260, formed to be on a same side as the input and output conductor patterns 250 and 260, and disposed to be parallel to the strip line 253 and a capacitor electrode 272 which is connected to one end of the strip line 271. Here, only the capacitor electrode 272 is disposed to face the earth conductor pattern 243.
The connection conductor pattern 280 is disposed by disposing dielectric ceramics 240a on a layer different from a layer on which the input conductor pattern 250, the output conductor pattern 260, and the resonance conductor pattern 270 are disposed. The connection conductor pattern 280 includes a capacitor electrode 281 which is disposed to face the capacitor electrode 252, a capacitor electrode 285 which is disposed to face the capacitor electrode 262, a strip line 282 of which one end is connected to the capacitor electrode 281, a connection electrode 283 which is connected to the other end of the strip line 282, and a strip line 284 of which one end is connected to the connection electrode 283 and the other end is connected to the capacitor electrode 285. In addition, the connection electrode 283 is connected to one end of the strip line 271 through a via conductor 273.
The filter circuit which is formed in the multilayered substrate 240 as illustrated in FIGS. 21, 22, and 23 is as shown in FIG. 24. In the aforementioned structure, components of the equivalent circuit shown in FIG. 24 will now be described.
A transmission line 293 connected to an input terminal 291 is constructed by the strip line 251, and a capacitor 221 is constructed by the capacitor electrodes 252 and 281. In addition, a transmission line 213 is constructed by the strip line 282.
In addition, a transmission line 294 which is connected to an output terminal 292 is constructed by the strip line 261, and a capacitor 222 is constructed by the capacitor electrodes 262 and 285. In addition, a transmission line 214 is constructed by the strip line 284.
In addition, a series resonant circuit including the transmission line 215 and the capacitor 216 is constructed by the strip line 271, and the capacitor electrode 272, and the earth conductor pattern 243.
In addition, a series resonant circuit including a transmission line 295 which is connected to a junction of the transmission line 293 and the capacitor 221 and a capacitor 297 is constructed by the strip line 253, the capacitor electrode 254, and the earth conductor pattern 243. In addition, a series resonant circuit including a transmission line 296 which is connected to a junction of the transmission line 294 and the capacitor 222 and a capacitor 298 is constructed by the strip line 263, the capacitor electrode 264, and the earth conductor pattern 243.
The frequency characteristic of the filter circuit 290, that is a BPF, in the multilayered substrate 240 constructed as described above is as shown in FIG. 25. In the figure, a curve A is an attenuation characteristic curve, and a curve B is a reflection amount characteristic curve. In addition, a vertical axis represents an attenuation amount and a reflection amount in units of dB, and a horizontal axis represents a frequency in units of GHz. As the attenuation characteristic curve A in the figure indicates, the attenuation amount has local maximal values at frequencies of 1.8 GHz and 5.2 GHz. In addition, as the reflection amount characteristic curve B in the figure indicates, the reflection amount has local minimal values at frequencies of 7.1 GHz and 9.6 GHz. The filter circuit 290 is designed on the basis of the filter circuit 220 described above to have a pass band of 6 to 10 GHz.
In order to implement the filter element 230 shown in FIGS. 17 and 20, the strip lines 251 and 253 and capacitor electrode 254 which are included in the input conductor pattern 250 shown in FIGS. 21 to 23 and the strip lines 261 and 263 and the capacitor electrode 264 which are included in the output conductor pattern 260 shown in FIGS. 21 to 23 can be removed.
The resonant circuit 210, the filter circuit 220, the filter element 230, and the multilayered substrate 240 which are described above are according to a general embodiment. Accordingly, as a different type of a BPF which can be used in another embodiment, a BPF which is disclosed in JP A-10-126104 is known.
However, as in the general embodiment described above, when the band pass filter (BPF) is constructed by using one resonant circuit, there is only one matching frequency at which the reflection amount becomes an extreme value, and accordingly it is difficult to maintain low insertion loss in the broadband. Although the number of matching frequencies may be increased by using a BPF which is formed by configuring multiple stages of the aforementioned resonant circuit, the configuration of multiple stages of the resonant circuits results in an increase in a circuit size and insertion loss. In addition, as shown in FIG. 25, in the BPF which is constructed by multiple stages of the resonant circuits, at an upper cutoff frequency which is in the proximity of pass band, an attenuation pole does not appear, and accordingly the BPF has a characteristic close to a substantial high-pass filter.